A mullet landing a marlin is when the mullet (guy who is highly unattractive) in spite of the odds being stacked against him, lands a marlin (hooks up with an extremely hot woman). This phenomenon is comparable to catching a marlin and pictures are usually taken for proof and bragging rights handed out accordingly.
Joe: dude, Rick is such a fucking mullet. He has no idea how to talk to a woman.
John: Bro, Rick landed a marlin last week. have some respect.
Joe: Holy shit, a mullet landing a marlin. Thats fucking crazy